1. Field of the Invention
The object of the present invention is to provide a process for the manufacture of soluble organic copolymers, which give non-tacky, water-resistant and elastic films on very diverse substrates or are suitable as binders for pigments, paints and non-wovens, and for the use of these copolymers as a binder component, the sheet-like structures, which can be manufactured from these polymers, having, however, properties which are considerably improved in various respects. In this connection, a lacquer, which contains the new soluble organic copolymers in combination with organic polyisocyanates and is applied to a metallic substrate, should cure, after drying in air for three to four days at temperatures of about 20.degree. C., in such a way that the film no longer swells under the action of water and also cannot be removed mechanically by scraping, even if the film is briefly exposed to water at temperatures of 50.degree.-70.degree. C.
Furthermore, the binders should exhibit a good pigment-binding capacity and should also possess good resistance to weathering. These binders are employed mainly as automotive lacquers for final lacquering or as car touch-up lacquers.
2. Prior Art
Numerous proposals have been disclosed for the manufacture of lacquers which are resistant to solvents and alkali by the reaction of polyisocyanates with copolymers containing hydroxyl groups and for the processing of these lacquers to coatings. In German Auslegeschrift No. 1,247,006 a process is described for the manufacture, by the polyisocyanate poly-addition process, of alkali-resistant sheet-like structures, which are obtained from copolymers containing hydroxyl groups and polyisocyanates, but these sheet-like structures are not adequately water-resistant after a reaction time of three to four days at 20.degree. C. These coatings obtained by this known process are therefore not suitable as lacquers for exterior lacquer coatings which are resistant to weathering, since blisters form after an extremely short time and the adhesion of the lacquer to the various metal substrates decreases and, as a result, the lacquer lifts off from the substrate.
It is also known that polyhydroxy compounds of polyols and polyisocyanates can be cured in order to obtain cross-linked lacquer coatings which have good resistance properties. These combinations also have too low a resistance to water and the resistance to weathering of the cured lacquer layers is relatively low.
In French Pat. No. 1,556,309 a coating agent is described which contains solvents and, as film-forming constituents,
A. a copolymer consisting of
(1) 2-50 parts by weight of an addition product, in the ratio of 1:1, of a carboxyl group of an .alpha.,.beta.-ethylenically unsaturated acid and an epoxide group of a glycidyl ester of an aliphatic carboxylic acid which has tertiary C atoms and in which the aliphatic tertiary group contains 4-26 C atoms, and
(2) 98-50 parts by weight of unsaturated copolymerisable monomers, the total amount of (1) and (2) adding up to 100 parts by weight, and
B. at least one polyisocyanate which contains at least 2 isocyanate groups in the molecule, the amount of polyisocyanate contained in the coating agent being, relative to the copolymer, 0.2-5 equivalents, based on the hydroxyl groups of the copolymer.
As the examples in Table 1 on page 5 of this French Patent Specification show, the copolymers described in this specification have a hydroxyl group content of 0.8 to 3.18%, based on the copolymer.
The known coating agents do indeed give films which have a hard, solvent-resistant and water-resistant surface. However, a decisive defect is the inadquate gloss, the extensive dulling after weathering and a very short processing time for the two-component lacquers due to too rapid gelling.
The solution of the copolymer itself gives very high viscosities, so that lacquers are obtained which have a very high solvent content and only a low solids content.
In French Patent No. 1,390,572 a process for the manufacture of copolymers is described in which an .alpha.,.beta.-unsaturated carboxylic acid, or one of its derivatives, is copolymerised, in a solvent, with one or more other vinyl monomers in the presence of a catalyst and, at the same time or subsequently, the reaction product is reacted with a glycidy ester of the formula: ##STR1## in which R is an aliphatic or aromatic radical which can also be substituted and which either is saturated or contains one or more ethylenically unsaturated bonds.
As the examples of this French Pat. No. 1,390,572 show, the copolymers contain several acrylates or mixtures of acrylates and methacrylates or several methacrylates in addition to styrene.
Moreover, it is mentioned in the resumee of this French Patent Specification, under letter D, that the copolymers described in the specification can be used for coating agents, which have been obtained by mixing the copolymer with an isocyanate prepolymer or a polyisocyanate. The coating agents of the type described in this specification also exhibit a relatively high elasticity after curing and therefore can still be attacked by solvents.
Copolymers which contain hydroxyl groups, are synthesised from monoethylenically unsaturated monomers and have an acid number of up to 40 and contain at least 5 per cent by weight of ethylenically unsaturated carboxylic acid units having active hydrogen atoms and which possess a structure consisting of the selected groupings, which comprise ##STR2## and ##STR3## wherein R is a tertiary aliphatic hydrocarbon radical having the structure ##STR4## wherein R.sub.1 represents --CH.sub.3 and R.sub.2 and R.sub.3 represent alkyl groups with 1 to 12 carbon atoms, and in which, according to claim 6, the monoethylenically unsaturated monomers are styrene and acrylic acid and the polymer has an acid number of 5 to 40, have already been described in U.S. Pat. No. 3,330,814.
These known copolymers can be used on their own as baking coating agents (loc. cit., column 1, lines 56-61). A combination of these copolymers with aminoplast resins (loc. cit., column 3, lines 52-62) and with phenol-formaldehyde resins (loc. cit., column 3, lines 63-66) is also mentioned and, in addition, epoxy polyethers (loc. cit., column 3, lines 67-75) can also be used. These coating agents are baked for 30 to 120 minutes at 121.degree. to 205.degree. C. However, the use of these copolymers together with polyisocyanates is not considered. U.S. Pat. No. 3,330,814 corresponds to British Pat. No. 1,009,217.
Copolymers of styrene, .alpha.,.beta.-ethylenically unsaturated monocarboxylic or dicarboxylic acids and glycidyl esters of .alpha.-ethylalkanemonocarboxylic acids are described in German Offenlegungsschrift No. 2,065,770.
The binder combinations of copolymers containing hydroxyl groups and polyisocyanates give very good water-resistant films; which are resistant even towards detergent solution, and, in addition, have an adequate processing time. However, these combinations have the defect that, in the case of pigmented lacquers containing titanium dioxide or pigment carbon black, the resistance to weathering is not adequate for automotive lacquering. The surface hardness and resistance to solvents of the films are also inadequate. The hydroxyl group content of these known copolymers is given as 1.5 to 2.8%.